Electrical devices, such as bushings, cable terminations, and measurement transformers, and capacitors may comprise capacitive electrical devices.
Capacitors, for example, may comprise a conductive layer wound around a shaft together forming a capacitive electrical device. The conductive layer is insulated from another conductive layer by means of an electrical insulator such as an insulating film.
Bushings may comprise a condenser core forming the capacitive electrical device. The condenser core comprises a plurality of conductive layers electrically insulated from each other and extending longitudinally along the electrical device body. In order to improve the electrical withstand strength, the axial length of each conductive layer is shorter than that of the corresponding insulation layers between which a conductive layer is sandwiched.
For large electrical devices the width of the films forming the electrical insulation available on the market may be less than that required by the electrical device. In this case it may be necessary to joint several films or sheets forming the electrical insulation of the capacitive electrical device. One known method of jointing insulating films is by winding films together with an overlap. This overlap creates areas of double film thickness at the joint. Further, this creates areas with high risk of voids and cavities in the vicinity of the overlap joint. This in turn reduces the dielectric strength in the joint area and/or may create partial discharges in service. Such a lap joint will make it difficult or impossible to make a sufficiently homogeneous condenser core of laminated films.
Another method that is utilised today for jointing involves placing papers or other porous films with an overlap and cut the paper in the overlap and removing the excess material, then impregnate the condenser core with an impregnation liquid in a vacuum impregnation process that fills up the cavities, inhomogeneities, and clefts with the fluid. The fluid can also be of the type that solidifies by a curing process, e.g. a resin. This method requires a substantial amount of time for the impregnation and curing process, which increases the manufacturing costs.
GB 1 129 995 A discloses a method of manufacturing an electric insulating bushing. The bushing is made by winding side-by-side on to a former or the like at least two sheets of flexible insulating material, e.g. paper, which initially overlap a distance W and are then cut by a reciprocating cutter in this overlapping region as they are fed forward together on to the former to produce complementary abutting serrated or tapered edges. Layers of foil may be interleaved with the insulating sheeting to reduce electric stress. The insulation may be increased by impregnating the bushing material with, for example, an insulating oil or gas, or a plastics resin that may be cured during fabrication of the bushing by using heated rollers.
DE 27 57 256 A1 discloses coaxial insulation for a cable and uses a signal inner conductor, or a rigid inner tube round a flexible inner conductor, as a winding mandrel for insulation tape, or web. Several such insulating webs are continuously applied to the winding mandrel in overlapping manner. The overlaps of two adjacent webs are so trimmed, prior to contacting the winder, as to form a narrow gap, between two adjacent webs. The winding mandrel and the web cutting device can be axially shifted, when one of them is stationary. Thus the gaps in individual lengths are covered. Alternately gaps in several layers may be covered by the top one.
GB 1 339 259 A discloses a method of manufacturing a bushing where two lengths of insulating sheet material are wound around a former. This document aims to manufacture a bushing in which the axial length of the insulator substantially exceeds the width of the paper or other insulating sheet material used. The figure below shows that the two pieces of insulation 11 and 12 are arranged in an overlapping manner forming overlaps 15 and 16, wherein a cutter 17 cuts the overlapping portion as the insulation is rolled onto the former 13. In this way the adjacent obliquely severed straight edges of the two lengths of paper will be laid in edge-to-edge abutted relationship, forming a butt joint of helical spiral form and of constant axial pitch. The completed bushing insulator may be impregnated may be impregnated by a vacuum process with a liquid casting resin, which when cured will bond the turns of the paper winding together.